Rework ignore to error when bug is open instead

[guibou]

Right now we designed ignore so a closed bug does not trigger an error. I think we should rework it so now an open bug triggers an error.

I see two reasons to add a link to a bug in a codebase:

• either it is a workaround for an issue and you want to be told when the linked issue is closed so you can remove your workaround.
• or it is a link to an issue which discuss and justify the current implementation. Most of the time this issue is closed. This is the main reason for the inclusion of the ignore feature.

However, if a new issue arise, a previously closed bug may be reopened and I'd like to be notified if a place in my codebase is making reference to this issue: there may be something to do there.